REPORT OF THE DIRECTOR I9IO 1 69 



also; and a much longer specimen (fig. 102b) which was less gib- 

 bons, with fairly strong depression beneath the beaks producing a 

 corresponding sinuosity in the ventral border. In this genus it 

 seems that these specimens must be regarded as being specifically 

 distinct. The first is taken as the type of his species and the 

 second (2820, in part, Peter Redpath Museum) is regarded as 

 belonging to the species now under discussion. The casts from 

 Grindstone island are smaller and show no trace of radiate mark- 

 ings, nor does the specimen (a cast) just referred to in the Dawson 

 collection. They are regarded as belonging to the same species. 



This species is very closely related to P. obsoletus Meek 

 and Worthen, from the Coal Measures of the Mississippi valley, 

 but is smaller, has the beak extending more sharply over the 

 cardinal area, and the long teeth parallel to the hinge reach much 

 farther back. It differs from P. cochlearis (Winchell) as 

 figured by Weller in that the posterior margin joins the hinge much 

 more nearly at right angles, making the shell less oblique. That 

 shell is probably the nearest relative known of our species. 



Leptodesma borealis nov. 



Cast of right valve small, aviculiform, with long projection in 

 front of the beak. Hinge about as long as the shell; posterior 

 margin sinuate above but soon becoming gently con- 





vex and gradually rounding into the ventral margin. 



Ventral margin quite sinuate beneath the beak on . t'T' k 



<=>^ Leptodesma bo- 



account of the strong depression in the shell, ca^st^o/ dght°vkive! 

 beyond which the border is convex to the tip odndstone l ^°"' 

 of the hinge. The umbonal ridge nearly dies out posteriorly. 

 Surface marked with varices of growth and smaller striae. 



Dimensions. Length of hinge, 8 mm ; length of umbonal ridge, 

 6 mm ; beak, 2+mm from front of hinge ; height of shell at posterior 

 end of hinge, 4.75 mm; angle of umbonal ridge to hinge about 30°. 



Liopteria dawsoni uov. 



Cast of left valve small, moderately convex, hinge shorter than 

 the length of the umbonal ridge. Posterior margin sinuate, rather 

 evenly rounded at the termination of the umbonal ridge; ventral 

 margin somewhat sinuate anteriorly but convex around the lobe 

 projecting in front of the beak. The lobe is small and nearly tri- 



